1. Field of the Invention
The present invention relates to active imaging systems in general and in particular to an active imaging system comprising a laser transmitter and an optical receiver wherein the receiver comprises a Faraday filter and a bare (non-intensified) CCD camera.
2. Description of Related Art
Radar has long been used for precision tracking of objects in space. However, in certain circumstances the use of radar for this purpose is found to have a number of disadvantages. For example, the resolution of a given radar is a function of its wavelength. As the wavelength is shortened the resolution is increased. Thus to increase the resolution of a radar beyond that achieved heretofore, the use of lasers to illuminate, image and track targets has been proposed as an alternative.
Heretofore, proposals for active imaging with lasers in space have in general comprised the use of a laser in combination with a dielectric optical filter and an intensified CCD camera. Among the disadvantages of such systems is the fact that dielectric optical filters are, with respect to the bandwidth of a laser at its operating frequency, relatively broadband devices and that intensified CCD cameras have a rather low quantum efficiency, e.g. 10%, compared to 80% or 90% for bare (unintensified) CCD sensors. The justification for using the dielectric optical filter and the intensified CCD is that the intensified CCD can be gated to reduce the frame integration time of the CCD down to a small number of nanoseconds, e.g. 10-30 nanoseconds, and thereby suppress the background of the sunlit earth, oceans, clouds, etc. However, this approach in turn requires that the illuminating laser be pulsed and operated in the visible region of the spectrum. For long range imaging, the only pulsed visible laser with sufficient power, e.g. 1.7 kilowatts (kw), to provide the required target irradiance is the frequency-doubled, Q-switched neodymium-doped yttrium aluminum garnet (Nd:YAG) laser operating at 532 nanometers (nm).